400mm Focus Trap

Well, yes. This year is shaping up to be more photography oriented as far as this blog goes. There will be other posts on other topics, I’ve not set aside all my interests yet. It’s just that this year is starting out with me playing around with my camera and lenses and getting re-acquainted with features I’d either forgotten about or hadn’t used yet. Also, those of you that have been following me for some time now know that I sort of lost interest in anything other than casual photography when a close friend died.

So, this post continues my playing with my 400mm f/5.6 Tokina prime lens.

From my last blog entry, you may have discerned that I was becoming a little…let’s say discouraged with my beloved 400mm lens. It was the first “real” lens I purchased after I got into photography back in the film ages. It worked great then, perhaps in part due to the split focus screen in my film cameras I used it with. Lately, though, attempting to use it was frustrating. A perfect lens for distant subjects like birds, it was difficult to focus on moving targets, and indeed even on still targets at times, due to the very narrow depth of field.

Then I discovered a trick called focus trap. It had merit, and it seemed like maybe it’d help, so I decided to grab the 400mm, go outside, and try it out.

Pictures taken at a focal point near the upper end of the lens were still blurred, but my belief is that’s due to trying to shoot the dang thing handheld on a very cold day. At that range, over 300 feet away, the slightest shake or vibration was magnified horrendously, even with shake reduction turned on. It didn’t help that I was also adjusting the focus as I was taking the photo. Try keeping the handheld camera still while doing that.

I was once again shooting birds roosting in a distant tree, and have no idea if the branches were swaying slightly as well, though it didn’t seem they were. However, these pictures of what appeared to be rather fat doves were much more sharper than I’d expected them to be. Not suitable for posting, but you could actually see the birds coloring and features, though you couldn’t make out individual feathers very well.

So maybe this lens isn’t dead yet.

I was surprised how well that focus trap worked out. It’s also what contributed to the improved images of the distant birds. Having tested it on the birds, I also tested it on several closer (about 20 feet) and mid-distance (about 120 feet) objects. With those, the images were nice, sharp, and detailed, everything I’d expected from this lens.

That leaves two possibilities:

     1) The upper end of the lens’ focus range can’t be used with my DSLRs 
        with any hope for sharp images.
     2) The upper end of the lens’ focus range can’t be used with my DSLRs

Of the two, right now I prefer to believe the second is the answer. I ran out of time before I could test it on the tripod. I hoped to try it out before posting this, but alas, didn’t manage to do so.

So, what is “focus trap”?

I’m not going to go into detail how to set up for this, there’s too many good, well-written tutorials and articles on the internet that do it better than I could.

Used with manual focus lenses, it’s a surprisingly simple technique that leverages some of the settings of the camera itself to detect the focus and take the picture at that very instant. It’s not meant to be used with lenses capable of auto-focus, because you have to use the auto-focus setting rather than the manual focus setting to get the camera to detect and take the photo.

It’s going to be very interesting, to say the least, to try this out on a moving target. I’m looking forward to that challenge, but from the above I know it won’t work with the upper end of my 400mm. But I’ll be trying it anyway. But before then, I’m going to practice it quite a bit on subjects that stay put for me long enough. When it’s second nature, then I’ll go after moving targets.

So, focus trap…how do you do it? You’ll need to set some of your DSLR’s custom settings for this to work, although it might work with the defaults with your camera. Try it without changing any settings first and see if it works. If not, then google “brand focus trap” where brand is your DSLR’s brand, i.e. for me “Pentax focus trap”. I knew I’d seen it before wrt Pentax manual lenses and so knew including Pentax would find specific guidelines for my camera. If “brand focus trap” doesn’t work, try taking the brand out.

The basics of focus trap:

     Mount your old manual focus lens on your camera.
     Set your focus to AF.S or whatever it is that lets you auto-focus
         for a single picture. Do not use AF.C which is for continuous
     After doing these two, when you turn on your camera you may be
        prompted to tell it what focal length the lens is. My camera does
        prompt me but I don’t know about others.
     Look through the viewfinder and verify your subject is out of focus.
     If your shutter button is set to use the half-press to focus, press
        the shutter button all the way down and hold it there. Your camera
        should refuse to take the picture unless it gets a focus lock.
     If your shutter and auto-focus button are separate, press and hold both.
     Slowly adjust your lens’ manual focus to bring your subject into focus.
     Release the shutter button, and the auto-focus button if separate, when
        the picture is taken.

Try to stop as soon as you get a focus lock, as indicated by your in-camera display but don’t worry if you go a hair past. The camera will automatically take the picture as soon as you have a lock. Obviously, it’s better if you can stop as soon as you see the focus lock especially since with slower shutter speeds you will take the image out of focus before the picture is completely taken if you keep adjusting.

Once you’ve done this at least one time, you’ll see that it’s really easier done than said.

There are, of course, some other things you have to set up, just as for any manual photography: shutter speed, aperture, ISO, etc.. Here’s some additional settings I use:

     I set my mode to Av, aperture priority. My manual lenses have an
        aperture ring so I will be controlling the aperture there.
     Usually, I set my aperture to wide open. That’s f/5.6 with my 400mm.
     I use center spot focusing.
     I have my Shake Reduction set to ON.
     I set my Custom setting to allow the use of the aperture ring on the
     I use the green button on my DSLR to grab the shutter speed from the
        program line.

As mentioned previously, I don’t know what these settings would be in other camera brands, but there should be something very similar. Your manual is your friend here.

Some of the articles mention that you should try approaching focus lock from both sides of the sharp area, i.e. both from a close focus out and from a far focus in closer. Apparently with some lenses, focusing from one or the other side works best. With my 400mm, in the conditions I was at the time, it appears that for me the result is the same from either side of the in-focus area.

I think it’s a useful trick to have in your repertoire and really appreciate its given me back my beloved 400mm. Maybe it’ll let you use those old manual lenses you have with your DSLR.

I’m excited to get back out there with the tripod and the 400mm.


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2 Responses to “400mm Focus Trap”

  1. Beryl O. Lindsay Says:

    Your question answers itself. One will focus automatically, generally as soon as you press the picture-taking button half-way down, and the other one will not. The lens without auto-focus is called a manual focus lens. You have to look carefully through the camera viewfinder (or at the LCD) and rotate the lens barrel until the image appears to be sharp and in focus to you and then take the picture. Auto-focus lenses CAN be used in the manual focus mode, but manual focus lenses do not offer an auto-focus mode.


  2. Bill Says:

    Hi Beryl,

    Thank you for your comment.

    The question wasn’t about manual focus vs. auto-focus, but rather about how to improve my focusing with my old 400mm manual lens. The article describes how I found a way to handle that using the AF feature of the camera body with a manual focus lens, the Focus Trap.

    This particular fully manual 400mm lens is very difficult to focus accurately when using my DSL bodies, particularly on a moving target, because the depth of field is so narrow. A still target, like a landscape, I can work with very slight movements of the manual focus ring on the barrel over the range of what appears to be nice and sharp through the viewfinder. But distant birds or animals, for example, or a landscape shot where the light is rapidly changing…focus trap as described in the article is almost the only way to go with this particular lens when time is of the essence or the target is moving.

    You’re right, many if not all auto-focus senses CAN be used in manual focus mode, but in this case an auto-focus lens won’t work with, and indeed does not need to resort to, the focus trap method. Since the focus trap takes advantage of the in-camera recognition of when the picture is in focus, with an auto-focus lens it would want to control the lens. That means that while you are trying to manually focus the auto-focus capable lens, so is the camera.

    Focus trap is a method that works well with fully manual lenses, once you get the hang of using the focus trap.


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